Radio Frequency Identification (RFID) transponders (tags) are generally operated in conjunction with RFID base stations (RFID readers) for a variety of inventory-control, security, and other purposes. Typically, an item having a tag associated with it, for example, a container with a tag placed on or inside it, is brought into a “read zone” established by the base station. The RFID base station may transmit an interrogating RF signal that is modulated by a receiving RFID tag. For example, the RFID base station may generate a continuous wave (CW) electromagnetic disturbance at a certain (carrier) frequency, and this disturbance may be modulated by the RFID tag to correspond to data that is to be communicated via the disturbance back to the base station. The modulated disturbance, or signal, may communicate the information at a rate, referred to as the data rate, which is lower than the carrier frequency. The RFID tag may modulate the RF signal in order to impart to the signal information stored within the RFID tag and may then transmit the modulated, answering, RF signal to the base station.
RFID tags may be active, containing their own RF transmitter, or passive, having no transmitter. Passive RFID tags, i.e., tags that rely upon modulated back-scattering to provide a return link to an interrogating RFID base station, may include their own power sources, such as a batteries, or they may be “field-powered,” whereby they obtain their operating power by rectifying an interrogating RF signal. Although both battery-powered and field-powered tags have minimum RF field strength read requirements, or read thresholds, in general, the range of a field-powered passive RFID tag may be limited by the amount of radiated power supplied from the RFID reader, and the range of an RFID tag having its own power source is limited by the signal to noise level of the total communications link. Because the interrogating signal must provide power to a field-powered passive RFID tag, the read threshold for a field-powered passive RFID tag is typically substantially higher than for an active RFID tag. However, because field-powered passive tags do not include their own power source, they may be substantially less expensive than active RFID tags, and because they have no battery to “run down,” field-powered passive RFID tags may be more reliable in the long term than active RFID tags.
Although field-powered passive RFID tags provide cost, reliability, and environmental benefits, there may be obstacles to the efficient operation of field-powered passive RFID tags. In particular, it may be difficult to deliver sufficient power from a base station to a field-powered passive RFID tag via an interrogating signal. The amount of power a base station may impart to a signal is limited by a number of factors, for example, regulations by the Federal Communication Commission (FCC) regarding signal frequencies and power. Consequently, it would be highly advantageous to provide an RFID tag that may be compact, lightweight, low cost, disposable, and that efficiently employs a substantial portion of the signal energy that it receives from an interrogating base station.
It is against this background that the miniature RFID tags described herein have been invented.